The present invention relates to a solid-state switch for selectively coupling an antenna to transmitting and receiving circuits, and to a type of field-effect transistor useful in this switch and other switches.
The antennas of time-division multiple-access (TDMA) digital wireless telephone sets, for example, must be switched rapidly between transmitting and receiving functions. Solid-state switches comprising diodes or transistors are commonly employed, as electromechanical switches would be too slow. In particular, a bridge network of field-effect transistors can be used as an antenna switch. The network acts as a double-pole double-throw (DPDT) switch, coupling the transmitting and receiving circuits alternately to the antenna and to ground.
To avoid loss of transmitting power, and to provide adequate coupling to ground, the field-effect, transistors in such a switch must have a low resistance in the on-state. To prevent leakage of transmitted signals into the receiving circuit, however, the transistors also need to have a very high resistance in the off-state. These are conflicting requirements, since a low on-resistance comes from a short channel, while a high off-resistance normally demands a long channel. To isolate receiving and control circuits from the high power output by the transmitter, the transistors must also have high source-drain and gate-drain breakdown voltages, but this too normally requires a long channel, which is incompatible with a low on-resistance.